1. Field of the Invention
The present invention relates to a monopole antenna capable of implementing a multiple-input multiple-output (MIMO) system, and more particularly, to a monopole antenna capable of implementing an MIMO system, which may be compact in size and reduce interference between antennas upon MIMO system implementation.
2. Description of the Related Art
Conventional handheld terminals have an external antenna installed therein, and the external antenna may include such as a monopole antenna, helical antenna, and so on.
The monopole antenna is formed of a conductive rod, and the length of the antenna is determined based on a frequency band. Thus, the length of the antenna becomes longer than the handheld terminal since the antenna should have a certain length maintained regardless of the size of the handheld terminal. Further, the antenna can be damaged due to external shock since the antenna is externally exposed.
The helical antenna is formed of conductive coil wound around a conductive plate. The helical antenna has an advantage in that it can be configured to be shorter as compared to the monopole antenna, but also can be damaged due to external shock.
Such an external antenna is placed on the head of a user when the handheld terminal is used, so the user can be adversely affected by electromagnetic fields.
In order to overcome the drawback of such an external antenna, the inverted F antenna (PIFA) has been proposed.
FIG. 1A is a perspective view showing a related art inverted F antenna, and FIG. 1B is a cross-sectioned view of the inverted F antenna of FIG. 1A. As shown in FIGS. 1A and 1B, the inverted F antenna is three-dimensionally formed with a ground part 10, a radiation part 12, a connection part 14, and a power supply part 16.
The radiation part 12 is disposed on the upper side of the ground part 10, and the connection part 14 is disposed on the end portion of the radiation part 12, and thus connects the ground part 10 to the radiation part 12. The power supply part 16 supplies electric current to the radiation part 12. In general, the impedance matching is determined based on the location of the power supply part 16 and the length of the connection part 14. The size of such a conventional inverted F antenna approximately becomes 15 mm×15 mm×6 mm.
Such an inverted F antenna can be built as an internal antenna in a handheld terminal, considerably resolving the drawback and providing easier fabrication as compared to the external antenna.
However, the conventional inverted F antenna has a problem in that there exists limitations to compactness and light weight in respect to the interval and sizes of the radiation part and the ground part. Further, the conventional inverted F antenna has a problem since a complicated fabrication or production process is taken due to the structure of the ground part and the structure of a power supply part and a separate supporting member is needed to support the radiation part.
FIG. 2 is a perspective view showing an antenna disclosed in U.S. Patent Application Publication No. 2005-62654, entitled ‘Planar Inverted F Antenna (PIFA)’. As shown in FIG. 2, the antenna disclosed in the patent publication is constructed with the ground plate 25 and a pair of inverted F antennas 20, and selection and usage is made from one of signals received at each inverted F antenna 20, using diversity characteristics.
However, in the above patent publication, the pair of inverted F antennas 20 is arranged in a row so that the whole length of the antenna becomes long, which causes difficulties in arranging plural inverted F antennas. Further, since the interval between the inverted F antennas 20 is narrow, the interference between the inverted F antennas 20 inevitably occurs. In particular, a certain size of ground plate is needed for each of inverted F antennas 20, so it is impossible to apply the inverted F antennas 20 to small-scale terminals.
FIG. 3 is a perspective view showing an antenna disclosed in U.S. Pat. No. 6,573,866, entitled ‘Multi-Frequency Hidden Antenna for Mobile Phones’. As shown in FIG. 3, U.S. Pat. No. 6,573,866 discloses a multi-frequency antenna using an electric power supply manner of the coplanar waveguide antenna having ground parts on the same plane as the electric power supplying line. The antenna 34 is connected to the electric power supplying line 33 supplied with electric power from the electric power supplying point 35, and the pair of ground parts 31 and 32 is disposed on the same plane as and spaced apart in a certain distance from the electric power supplying line 33. Further, the flattened short circuit 37 connecting the electric power supplying line 33 with the ground part 32 is provided so that a frequency is determined depending on a location of the flattened short circuit 37, which enables multi-frequency implementation.
However, such a conventional antenna, like the other conventional antenna stated as above, has a drawback since it has to maintain the lengths of the electric power supplying line 33 and the antenna 34 over a certain length, and has difficulties in application to small-scale terminals due to the electric power supplying structure of the coplanar waveguide antenna.
Meanwhile, there has been proposed the MIMO antenna having plural antenna elements arranged in a special structure and performing multiple-input and multiple-output operations. The MIMO system combines radiation patterns of the plural antenna elements and combines radiation power of the plural antenna elements, to sharpen the shape of the whole radiation pattern and then have an electromagnetic wave travel much farther.
However, such an MIMO system requires many small antenna elements in order that the plural antenna elements are mounted in a small-scale terminal, which is very difficult in realization with the above conventional antennas.
Thus, much smaller antennas are required as small-scale terminals become more compact, and in particular, the structure of small antenna elements has to be proposed which can realize the MIMO system.